Question

I’m getting this code analysis error: CA1823:AvoidUnusedPrivateFields, for this line of code:

private static Timer _timer;

In the constructor of my class, I have this:

_timer = new Timer(OnTimerElapsed, _autoResetEvent, 1000, 1000);

I don’t need to call any methods on the timer; simply instantiating it is enough. Is that why I’m getting the CA warning? Is this a case where I really just need to suppress the error, or am I missing something?

As a side note, I’m not getting an error that I should be calling Dispose() on the timer. Shouldn’t I be getting that error?

— Edit —

If I make the _timer an instance field, then I get the error about not calling Dispose() on it. Why would static vs. instance matter for that?

Answer 1

You are getting the warning because you are not accessing the _timer variable after assigning to it.

The reason that the errors are different depending on whether or not the variable is static is exactly because of the nature of static variables. A static variable is an part of the type, not of any particular object. Its lifetime is tied to the lifetime of an application domain, and not of any particular object. Since its lifetime is tied to that of the application domain, the owner of the application domain is responsible for the timer’s cleanup. The owner of the application domain is the CLR itself, so the CLR will clean up your static timer. Static analysis does not complain that you do not call Dispose because the timer’s finalizer will be called while the app domain is unloaded.

If the timer were an instance field, its lifetime would be tied to that of the object which owns it. In that case, the timer’s cleanup would be the responsibility of the owning object. In C#, such object-member cleanup is performed using the Disposal pattern. If you haven’t implemented that pattern (by implementing IDisposable) VS static analysis will issue a warning.

Now that being said, timers are a special case of IDisposable. They are special because even if you are “using” the timer (by executing code in its callback), you are not accessing the timer object itself. Hence, you might be tempted to think of the Timer class as a service. That is, you might think that the Timer constructor is more like a RegisterForPeriodicCallbacks method. That is wrong. Timer is an object, and in C# you have to treat it as such. You must think of Timer as an object with lifetime, and one that is determined by some other object.

If you want the lifetime of the timer to be tied to the lifetime of the application, then you should make a static variable of type System.Timers.Timer rather than System.Threading.Timer. When you want the timer to begin ticking, simply call its Start method.

If you want the lifetime of the timer to be tied to the lifetime of some other object, then you should store it as a field in that other container object, and be sure to correctly implement the Disposal pattern on the container object.

If you do either of these things, all of the static analysis warnings will go away.